Occurrence and onset conditions of postsunset equatorial spread F at Jicamarca during solar minimum and maximum

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1 JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 115,, doi: /2010ja015650, 2010 Occurrence and onset conditions of postsunset equatorial spread F at Jicamarca during solar minimum and maximum Chien Chih Lee 1 Received 10 May 2010; revised 14 June 2010; accepted 1 July 2010; published 28 October [1] The data of digisonde and the NRLMSISE 00 model on the occurrence of postsunset equatorial spread F (ESF) at Jicamarca during solar minimum and maximum years are used in a detailed statistical analysis. The onset conditions chosen in this study are the prereversal enhancement velocity (dhf 80 /dt), minimum virtual height of F layer (h F), gradient density scale length (L), and local linear growth rate (g) of collisional Rayleigh Taylor instability. The monthly variation in ESF occurrence during solar minimum generally agrees with that during maximum. This agreement indicates that the ESF occurrence does not depend on solar activity. For the onset conditions, the differences between solar minimum and maximum exists in the monthly variations in dhf 80 /dt, h F, and L, but not in g. This consistence in monthly g variation between two solar epochs mainly account for the solar cycle independence of ESF occurrence. Furthermore, the variations in collision frequency and recombination rate are principally responsible to the consistence in g. Citation: Lee, C. C. (2010), Occurrence and onset conditions of postsunset equatorial spread F at Jicamarca during solar minimum and maximum, J. Geophys. Res., 115,, doi: /2010ja Introduction 1 General Education Center, Ching Yun University, Jhungli, Taiwan. Copyright 2010 by the American Geophysical Union /10/2010JA [2] Near the dip equator, the equatorial spread F (ESF) usually occurs after local sunset. The occurrences of ESF would vary with longitude, local time, season, and solar and geomagnetic activities [e.g., Fejer and Kelley, 1980; Abdu, 2001; Kelley, 2009]. However, in the Peruvian sector, the occurrence of the postsunset ESF seems to be independent of solar cycle. In Rastogi [1980], the Huancayo (12 S, 75.3 W) ionosonde data covering were applied to study the occurrences of the range (RSF) and frequency (FSF) types of spread F. It can be found in Figures 1 and 3 of Rastogi [1980] that the greatest value in the sum of RSF and FSF occurrences during solar minimum was close to that during solar maximum. Furthermore, Hysell and Burcham [2002] reported the ESF statistical results using JULIA (Jicamarca Unattended Long term Investigation of the Ionosphere and Atmosphere) radar at Jicamarca (12 S, 76.9 W), and indicated that the ESF occurrence does not have significant solar cycle dependence. This kind of independence seems to appear only in this longitudinal sector. Since the causes for the independence are not investigated yet, this work is the first attempt to study why the ESF occurrence in the Peruvian sector does not vary with solar activity. [3] In the Peruvian sector, many scientists have studied the occurrence and onset conditions of ESF. Farley et al. [1970] used the Jicamarca VHF radar to observe the F region irregularities, and suggested the F layer height and the prereversal enhancement (PRE) E B drift velocity are important factors for the ESF onset. Chandra and Rastogi [1972b] investigated the relation of the minimum virtual height of F layer (h F) to spread F at Huancayo for low, medium, and high solar activities. Fejer et al. [1999] used the observations of Jicamarca VHF radar during to reveal the effects of the PRE velocity on the generation and evolution of ESF. Hysell and Burcham [2002] examined the relationship between ESF and the PRE velocities under geomagnetic quiet and disturbed conditions. Whalen [2002] and Lee et al. [2005] studied the dependence of ESF occurrence on the PRE velocity during solar maximum using the ionogram data of Huancayo and Jicamarca, respectively. Lee [2006] used the data of Jicamarca digisonde and MSISE 90 model [Hedin, 1991] to estimate the local linear growth rate of collisional Rayleigh Taylor instability (CR T) during solar maximum, and proposed that the monthly variations in growth rate and ESF occurrence are similar. Recently, Chapagain et al. [2009] reported the climatological behaviors in altitude, time, and PRE velocity for ESF onset using the dataset of Jicamarca VHF and JULIA radars from 1996 to [4] On the basis of the previous works mentioned above and in other longitudinal sectors [e.g., Jayachandran et al., 1993; Mendillo et al., 1992; Manju et al., 2009], the onset conditions of ESF chosen in the present study are PRE velocity and h F, gradient density scale length, and local linear growth rate of CR T instability. These onset conditions are analyzed concurrently in the Peruvian sector for the first time. This study not only examines the occurrence and onset conditions of ESF during solar minimum and maximum, but tries to explain why the ESF occurrence is inde- 1of7

2 Table 1. The Monthly Values of Smoothed Sunspot Numbers (SSN) and Solar Flux (F10.7) January December 1996 April 1999 March 2000 Month (solar minimum) Monthly SSN Monthly F10.7 Month (solar maximum) Monthly SSN Monthly F10.7 January January February February March March April April May May June June July July August August September September October October November November December December pendent of solar cycle in this sector. To avoid the effects of geomagnetic disturbances, the data under geomagnetic quiet conditions are used in this study. 2. Data Analysis [5] The data of ESF, h F, the PRE velocity, and the gradient density scale length (L) are obtained from the ionogram data of Jicamarca digisonde (12 S, 76.9 W, geomagnetic latitude: 1.28 N). The presence/absence of ESF, the ionospheric parameters (e.g., h F, fof 2 (maximum plasma frequency of F layer)), and the true height of electron density profile are determined by both the SAO explorer [Reinisch, 1996; Huang and Reinisch, 2001] and manual work. The SAO explorer, developed by Center for Atmospheric Research, University of Massachusetts Lowell, provides scientists with ionogram data visualization, interactive ionogram scaling, and density profile inversion. The values of L and hf 80 are derived from the electron density profile. The definition of hf 80 is the height at the plasma frequency of f = 0.8 fof 2 [for detail, see Bertoni et al., 2006]. Since Bertoni et al. [2006] have showed that the dhf 80 /dt agreed with the vertical E B drift velocity of Jicamarca ISR radar during the postsunset period, the dhf 80 /dt is considered to be the PRE velocity in this study. [6] The local linear growth rate (g) used in this study is based on the equation of the CR T instability [Ossakow et al., 1979; Ossakow, 1981]: ¼ 1 n g R s 1 ; where n 0 is the ambient electron density, h is the height above the Earth, n is the ion neutral collision frequency, g is gravity (positive downward), and R is the local recombination rate. The (1/n 0 )( n 0 / h) is the inverse of gradient density scale length (L ). The n and R in equation (1) are given by [Strobel and McElroy, 1970] as ¼ 2: T 0:5 n n s 1 and R ¼ K 1 nðo 2 ÞþK 2 nðn 2 Þ s 1 ; where T is the atmospheric temperature in degrees Kelvin, n n is the neutral number density in cm 3. In equation (3), n(o 2 ) and n(n 2 ) are the neutral number densities of O 2 and N 2 in ð1þ ð2þ ð3þ cm 3.TheK 1 and K 2 in(3)aregivenby[mcfarland et al., 1973] as K 1 ¼ :4 ð4þ T K 2 ¼ 1: as T 750 K T ð5þ K 2 ¼ T 2 as T > 750 K: ð6þ 300 These atmospheric quantities in equations (2) (6) are obtained from the NRLMSISE 00 model [Picone et al., 2002]. [7] In this work, the maximum values ((dhf 80 /dt) max and h F max )ofdhf 80 /dt and h F, as well as the greatest value (g max ) in the maximum g profile between 18:00 LT and the onset time of ESF are applied to the statistical analyses. And, the values of n, R, and L associated to g max are defined as n max, R max, and L max respectively [for detail, see Lee, 2006]. The periods of the observation and model data for solar minimum and maximum are January December 1996 and April 1999 March 2000, respectively. Table 1 displays the monthly values of smoothed sunspot numbers (SSN) and solar flux (F10.7) for these two solar epochs. It is noted the 23rd solar cycle started in May 1996 with the monthly SSN of 8.0, and peaked in April 2000 with [8] In order to eliminate the influence of geomagnetic disturbances, this study focuses on the occurrence and onset conditions of ESF during geomagnetic quiet condition. The definition of geomagnetic quiet condition is that the average value of Kp recorded in the interval of UT (13:00 19:00 LT) is equal to or less than 3 (Kp 3), following the criterion of Fejer et al. [1999]. 3. Results and Discussion [9] Figure 1 displays the monthly variations in the occurrence probabilities of ESF during solar minimum (dashed line with solid circle) and maximum (solid line with diamond). The occurrence probability is the number of quiet condition days on which at least one ESF event is observed during 18:00 24:00 LT divided by the number of quiet condition 2of7

3 Figure 1. The occurrence probabilities of ESF under geomagnetic quiet condition during solar minimum and maximum. The dashed line with solid circle is the data during solar minimum; while the solid line with diamond represents the data during maximum. days for a month. In Figure 1, the ESF occurrences during solar minimum agree generally with those during solar maximum, except the difference is larger than 20% in May. The ESF occurrences are larger (77% 100%) in the E (March, April, September, and October) and D (January, February, November, and December) months, but smaller (23% 65%) in the J months (May August). This agreement indicates that the ESF occurrences do not depend on solar cycle, and confirms the point of Hysell and Burcham [2002]. On the other hand, this result is different from that of Chandra and Rastogi [1970], in which the spread F index at Huancayo decreases linearly with sunspot number. [10] The ESF occurrences during solar maximum were reported by Whalen [2002] and Lee et al. [2005]. In their studies, the monthly variations in ESF occurrences are close to the results of this study, although their ESF occurrences were not estimated only under geomagnetic quiet condition. Whalen [2002] and Lee et al. [2005] proposed that the monthly variation in ESF occurrence is related to the seasonal behaviors of the PRE velocity [Fejer et al., 1999]. Moreover, Lee [2006] indicated that the monthly variation in g max also accounts for that in ESF occurrence during solar maximum. For solar minimum, this kind of monthly variation in ESF occurrence is not studied yet. In the following sections, the onset conditions of ESF during solar minimum and maximum are analyzed. [11] In order to explore the mechanisms for the solar cycle independence of ESF occurrence, the onset conditions, (dhf 80 /dt) max, h F max, L max and g max, during solar minimum and maximum are examined. First, the monthly averages of (dhf 80 /dt) max are calculated and presented in Figure 2a. The monthly average is the average value of (dhf 80 /dt) max under geomagnetic quiet condition for a month. Although the PRE velocities for low and high solar activities have been addressed by Fejer et al. [1999], the period of their observation data did not cover January December 1996 and April 1999 March It is found in Figure 2a that the (dhf 80 /dt) max have two peaks at March and October for solar minimum (18 and 15 m s ) and maximum (57 and 50 m s ). The (dhf 80 /dt) max in the D months are m s for solar minimum, and m s for solar maximum. The smallest values existing in the J months are 8 10 and m s for solar minimum and maximum, respectively. It is noted in Table 1 that the values of monthly F10.7 are ( ) for the E months, ( ) for the D months, and ( ) for the J months during solar minimum (maximum). Overall, the (dhf 80 /dt) max values are greater during solar maximum than solar minimum. These variations in the (dhf 80 /dt) max are close to the results of Fejer et al. [1999]. In Fejer et al. [1999], for high solar activity (mean F10.7 equals 190), the peaks of PRE velocities are 48, 19, and 33 m s for the E, J, and D months, respectively; for low solar activity (mean F10.7 equals 85), the peak velocities are 14 and 6 m s for the E and D months, respectively. Furthermore, the similar results were also found in the Indian sector [Jayachandran et al., 1993]. Jayachandran et al. [1993] presented that maximum values of PRE velocity are 45 and 20 m s for January April 1984 (mean F10.7 equals 120) and January April 1985 (mean F10.7 equals 70), respectively. [12] Figure 2b shows that the monthly averages of h F max for these two solar epochs. The monthly average is the average value of h F max under geomagnetic quiet condition for a month. In Figure 2b, it is clear that there are two peaks at April (289 km) and October (282 km) for solar minimum, and at March (412 km) and October (408 km) for solar maximum. The lowest values of h F max are in the J months for both solar minimum ( km) and maximum ( km). For the D months, the h F max values are km and km for solar minimum and maximum, respectively. These are comparable to those found at Huancayo by Chandra and Rastogi [1972b], in which the peak values of h F are 430, 415, and 361 km (292, 315, and 261 km) for the E, D, and J months of solar maximum (minimum), respectively. In India and Africa, the h F values near the dip equator also increase with solar activity, found by Chandra and Rastogi [1972a; 1972b]. Moreover, the results in the present study also consist with the statistical results of radar observation. Hysell and Burcham [2002] found that the bottomtype layer height of ESF typically increases from about 200 to 400 km for an increase in F10.7 from 70 to 200. Chapagain et al. [2009] reported the onset height of ESF from 280 to 420 km in equinox, from 270 to 410 km during the December solstice, and from 250 to 400 km during the June solstice, as F10.7 index increases from 60 to 250. Additionally, it is found in Figures 2a and 2b that the monthly variations in dhf 80 /dt max and h F max are qualitatively close to each other. This illustrates that the height of F layer is mainly controlled by the PRE velocity during the postsunset period. [13] Next, the monthly averages of L max during solar minimum and maximum are displayed in Figure 2c. The monthly average is the average value of L max under geomagnetic quiet condition for a month. It is found that the greatest values are in July ( km ) for solar minimum and in May ( km ) for solar maximum. 3of7

4 Figure 2. The monthly averages of (a) (dhf 80 /dt) max, (b) h F max, (c) L max, and (d) g max under geomagnetic quiet condition during solar minimum and maximum. The dashed line with solid circle represents the data during solar minimum; while the solid line with diamond is the data during maximum. The smallest values exist in March for both solar minimum ( km ) and maximum ( km ). These values of L max ( km ) are close to that in Mendillo et al. [1992] ( km ), but smaller than that in Ossakow et al. [1979] ( km ) and Manju et al. [2009] ( km ). Furthermore, it can be found that there is not clear trend in the monthly L max variations for both two solar epochs. The result during solar maximum was also found by Lee [2006], although the criterion of geomagnetic quiet condition in Lee [2006] differs from that in this study. It is noted that the values of L max during solar minimum are greater than those during solar maximum. Manju et al. [2009] also presented that the L values are greater during solar minimum in the Indian sector. This might be because the density gradient is smaller [Jayachandran et al., 1993] and the ambient density is greater when the F layer is located at higher altitude during solar maximum. [14] The last onset condition examined in this study is g max. Figure 2d shows the monthly averages of g max for solar minimum and maximum. The monthly average is the average value of g max under geomagnetic quiet condition for a month. In Figure 2d, the monthly g max variations for both solar minimum and maximum are generally close to each other, except the difference is about s in January. The values of g max are greater ( s ) in the E and D months, but smaller ( s ) in the J months. The monthly g max averages for solar maximum have been reported by Lee [2006], in which the g max values are similar to those in this study. Moreover, the g max values are also corresponding to those in other studies. Ossakow et al. [1979] applied the linear growth rates of s on their simulation works. In the experimental investigations, Mendillo et al. [1992] and Sastri et al. [1997] calculated the growth rates from the observation and model results, and have the values of and s, respectively. Moreover, the flux tube integrated growth rates at Huancayo (12 S, 75.3 W) during solar maximum estimated by Sultan [1996] are s. 4of7

5 Figure 3. The monthly averages of (a) n max and (b) R max under geomagnetic quiet condition during solar minimum and maximum. The dashed line with solid circle is the data during solar minimum; while the solid line with diamond represents the data during maximum. [15] The previous studies showed that the PRE velocity and elevated F layer height during the postsunset period are prerequisite for the ESF onset [e.g., Farley et al., 1970; Mendillo et al., 1992; Jayachandran et al., 1993; Fejer et al., 1999]. In this study, during solar maximum, the monthly variations in (dhf 80 /dt) max and h F max can account for that in ESF occurrence during solar maximum, according to Fejer et al. [1999], Whalen [2002], and Lee et al. [2005]. However, during solar minimum, the smaller (dhf 80 /dt) max and lower h F max seems to be indirectly related to the ESF occurrence, which is similar to that during solar maximum. For the gradient density scale height, the earlier studies proposed that L might be an important parameter in controlling in the ESF occurrence, even though L is less significant than the PRE velocity and elevated F layer height [Ossakow et al., 1979; Jayachandran et al., 1993; Manju et al., 2009]. In the present study, the monthly variations in L max are neither correlated nor anticorrelated to those in ESF occurrence for both two solar epochs. This indicates that L max is not a major factor for the ESF onset, although L is proportional to g in equation (1). Regarding the growth rate, the monthly variations in g max are not only similar between solar minimum and maximum, but agreeable to those in the ESF occurrences. These coincidences demonstrate that the similarity in g max is mainly responsible for that in ESF occurrences between two solar epochs. [16] In order to investigate the similarity in g max between these two solar epochs, the collision frequency and recombination rate, which are the terms in (1), are analyzed. Figure 3 shows the monthly averages of n max and R max for solar minimum and maximum. For the monthly variations in n max and R max, the values of are greater in the J months, but smaller in the E and D months for both solar minimum and maximum. The n max values during solar minimum are s, which are greater than those ( s ) during solar maximum. Regarding the recombination rate, the R max ( s ) during solar minimum are also greater than those ( s ) during solar maximum. It is noted that the values of n max and R max during solar maximum are close to those in Lee [2006], in which this kind of monthly variation is related to the seasonal behaviors of PRE velocity. [17] In equation (1), the n and R are inversely proportional to the g. This relationship is coincident with the results, which are that the monthly variations in n max and R max are anticorrelated to that in g max. Therefore, the values of n and R are the major terms for the magnitude of g in (1). Further, the differences in n max and R max between solar minimum and maximum are examined. For instant, the average profiles of n and R at 20:00 LT in December under quiet condition are showed in Figure 4. The altitude of n max and R max is 297 km for solar minimum, and 450 km for solar maximum. It is evident that the smaller n max and R max are found during solar maximum, even if the values of n and R profiles are greater. These smaller values of n max and R max are because the F layer is lifted to higher altitudes by the greater PRE velocity during solar maximum. 4. Conclusion and Summary [18] In this study, the occurrences and onset conditions of ESF during solar minimum and maximum are examined to explore the mechanisms for the solar cycle independence of ESF occurrence. The data of Jicamarca digisonde and NRLMSISE 00 model under geomagnetic quiet conditions are applied to calculate the values of (dhf 80 /dt) max, h F max, L max, and g max, which are chosen to be the onset conditions. [19] The results show that the monthly variation in ESF occurrence during solar minimum is generally consistent with that during solar maximum. This consistence illustrates that the ESF occurrence does not vary with solar cycle. However, this kind of consistence is not found in the onset conditions of (dhf 80 /dt) max, h F max, and L max. The differences exist in the monthly variations in the three onset conditions between two solar epochs. On the other hand, the monthly variations in g max during solar minimum and maximum are generally similar to each other. This similarity indicates the g max is independent of solar cycle. Furthermore, the monthly variations in g max are corresponding to 5of7

6 B. W. Reinisch of the Center for Atmospheric Research, University of Massachusetts Lowell, MA, U.S.A. for the ionogram data of DIDBase, Community Coordinated Modeling Center (CCMC) for providing NRLMSISE 00 model, and the National Geophysical Data Center (NGDC) for providing data of Kp, F10.7, and sunspot number. [22] Robert Lysak thanks Ram Rastogi and Narayan Chapagain for their assistance in evaluating this paper. Figure 4. The monthly average profiles of (a) n and (b) R under geomagnetic quiet condition at 20:00 LT in December for solar minimum and maximum. The dashed line represents the data during solar minimum; while the solid line is the data during maximum. The values of n max and R max for solar minimum and maximum are marked by solid points. that in ESF occurrence for both two solar epochs. These results demonstrate that the solar cycle dependence of ESF occurrence is mainly related to that of g max. [20] Because the values of n max, and R max are greater during solar minimum than solar maximum, and the monthly variations in these two parameters are anticorrelated to that in g max, the solar cycle independence of g max is mainly caused by the variations in n max, and R max. Additionally, the variations in n max, and R max are affected by those in the PRE velocity and F layer height. This indicates that the (dhf 80 /dt) max, h F max are still significant to the magnitude of g max values. [21] Acknowledgments. This work was supported by the National Science Council NSC M MY3 and National Space Organization NSPO S (L). The author would like to thank Prof. References Abdu, M. A. (2001), Outstanding problems in the equatorial ionospherethermosphere electrodynamics relevant to spread F, J. Atmos. Sol.Terr. Phys., 63, Bertoni, F., I. S. Batista, M. A. Abdu, B. W. Reinisch, and E. A. 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7 Sastri, J. H., M. A. Abdu, I. S. Batista, and J. H. A. Sobral (1997), Onset conditions of equatorial (range) spread F at Fortaleza, Brazil, during the June solstice, J. Geophys. Res., 102, 24,013 24,021. Strobel, D. F. and M. B. McElroy (1970), The F2 layer at middle latitude, Planet. Space Sci., 18, Sultan, P. J. (1996), Linear theory and modeling of the Rayleigh Taylor instability leading to the occurrence of equatorial spread F, J. Geophys. Res., 101, 26,875 26,891. Whalen, J. A. (2002), Dependence of equatorial bubbles and bottomside spread F on season, magnetic activity, and E B drift velocity during solar maximum, J. Geophys. Res., 107(A2), 1024, doi: / 2001JA Chien Chih Lee, General Education Center, Ching Yun University, No. 229, Jiansing Rd., Jhungli, Taiwan. (cclee@cyu.edu.tw) 7of7